The gasoline engine may have a lower fuel feed pressure to an injection valve than that of the Diesel engine. Unlike the case of the Diesel engine, therefore, the fuel injection adopted is accomplished not by opening a needle valve by the pressure itself of the fuel but by lifting and opening a control valve by the electromagnetic force thereby to inject the fuel reserved in advance in a valve chamber. The amount of fuel injection is controlled by the time period for which the control valve is opened.
Thus, the fuel injection valve in this field is generally constructed by arranging a needle (or spindle) valve having a blunt leading end slidably relative to the nozzle body having an injection hole, by equipping the leading end of the needle valve with a liquid tight seat mechanism, by welding an armature to the rear portion of the needle valve, and by exerting a magnetic attraction upon the armature.
Moreover, the fuel injection valve of this kind is earnestly desired to have a finer atomization from the aspect of exhaust emission control. One fuel atomization is known in Japanese Patent Laid-Open No. 219350/1989.
This prior art is intended, by forming a short spindle-shaped sliding portion to contact with the cylindrical inner face of a valve body upstream of the seat portion of a needle valve and by forming a helical groove in that spindle-shaped sliding portion, to impart a rotational energy to the fuel passing through the helical groove thereby to establish a swirl when the needle valve is lifted.
Since, however, this prior art is formed with the helical groove all over the spindle-shaped sliding portion, the pressure is highly lost at that portion. Because the fuel pressure is low, moreover, the fuel will hardly diffuse even it flows helically, so that the injection characteristics are adversely affected. Since, on the other hand, the volume of the portion from the helical groove to the seat portion is large, the dead volume is inevitably increased to change the shape of atomization in dependence upon the period of the opening time of the needle valve. Specifically, at the beginning of the fuel injection, the helical groove itself forms part of the dead volume, in which the fuel has no rotational energy, so that the atomization takes the form of pencil stream.
Since, on the other hand, the aforementioned needle valve is lifted by the electromagnetic force, it is absolutely necessary for a proper injection of fuel that the needle valve is firmly joined to the armature. This joint structure is exemplified by a method of welding the tubular control valve or needle valve to the armature, as disclosed in the above-specified Japanese Utility Model Laid-Open No. 163171/1984.
In this prior art, however, it is necessary to machine and finish the external diameter of the tubular control valve and the internal diameter of the armature. Especially in case the laser welding is adopted as the welding method so that the radially inner edge of the lower end of the armature is welded on its whole circumference to the outer circumference of the tubular control valve, welding defects are caused, if the portions to be joined have a clearance, and still the worse the lift is dispersed. Thus, a strict sizing accuracy is required. As a result, there arises a problem that the production cost is extremely high. On the other hand, the lift of the tubular control valve is set in terms of the insertion position of the tubular control valve into the armature. In the prior art, however, the tubular control valve is press-fitted in the armature so that the fitting allowance is eliminated to produce defective articles if the needle valve is retracted in case the fitting size is excessively large. Thus, the prior art cannot be freed from the rise of the production cost and the reduction of the production yield.